Gas turbine engines include many types of actuators for positioning control members for controlling operation of the engines. The actuators may be linear or rotary servomechanisms having output members whose position is to be controlled. The actuators may be hydraulic, pneumatic, or electrical, and are used for various purposes. For example, a gas turbine engine includes actuators for controlling propeller pitch, positions of variable geometry stator vanes in the high-pressure and low-pressure turbines, and for controlling the position of fuel metering valves and afterburner variable area nozzles.
Closed loop feedback control systems for the actuators are well known in the art and provide automatic control for maintaining a desired or demand position of the actuators. The feedback control systems may be implemented in conventional forms such as an analog embodiment, a dedicated digital embodiment, and computer embodiments utilizing software, or algorithms.
For improving reliability and operation of a gas turbine engine used for powering an aircraft, fault detection systems are typically utilized in the engine control system for detecting faults and for allowing remedial action to be taken. It is conventional to use mathematical models of the entire control system, i.e., the particular closed loop feedback control system for a particular actuator, to predict the output of the actuator and compare that output of the actuator with the prediction to determine if any deviation therebetween is indicative of a fault occuring in the control system and actuator.
A mathematical model usually cannot exactly duplicate the characteristics of a control system and actuator, particularly during transient as opposed to steady state operation. A closed loop control system has a dynamics performance including both steady state and transient response. The dynamic performance of the system includes conventional factors such as stability, accuracy and response time. In order to have acceptable performance in a fault detection system having a mathematical model representative of the actuator control system, the fault detection system dynamics must be closely matched to the actuator control system dynamics both during steady state and transient operation.
Accordingly, relatively complex mathematical models are typically utilized for modeling the control system for improving the matching between the systems dynamics. Relatively simple models of the actuator control system will result in relatively large deviation between the actuator output and the predicted model output at least during transient operation of the system. Such relatively large deviation can lead to erroneous fault indications which may be reduced by increasing the threshold above which a fault indication is made. However, increasing the threshold reduces the sensitivity of failure detection by eliminating fault indications for failures below the threshold, as well as decreases the response time for indicating a failure.
The response time of the fault detection system is primarily determined by the transient dynamic operation of the fault detection system. In some applications in a gas turbine engine, very high response times are required in order to detect a fault in the system to allow for prompt remedial action. For example, a gas turbine engine includes a fuel metering control system having an actuator which positions a fuel valve for metering the amount of fuel to the engine, and thereby controlling the operation thereof. In an engine operating at 95% rated rotor speed, for example, a fuel metering valve actuator failure such as for example a fail-positive would allow for increased fuel flow to the engine and possibly lead to compressor stall or engine rotor overspeed. In such an exemplary situation, a response time of about 100 milliseconds or less is desired in order to sense the fail-positive fault and to take remedial action such as shutting down the engine if the actuator itself fails, or automatically changing over to a redundant circuit to the actuator if the primary control circuit fails. Accordingly, the transient dynamic response of the fault detection system must be sufficiently fast, sensitive, and reliable for detecting a fault for allowing sufficient time for taking remedial action.
Since improved transient performance of the fault detection system is typically obtained by increasing the complexity of the mathematical model used therein, matching of the mathematical model to the system dynamics becomes more difficult. In such situations, the fault detection system must be carefully tailored to the dynamics of the actuator control system for obtaining accurate modeling. However, if any changes are made to the actuator control system during design or during testing, the fault detection system must be similarly adjusted to match the system dynamics of the control system, which is a time-consuming process.